Defects in the individual picture element (pixel) cells that constitute a video imager, unless corrected, may cause defects in the output images produced by the imager. These defects may appear as bright spots or lines in an otherwise dark image or dark spots or lines in an otherwise bright image. Because video imagers typically include a large number of pixel cells, it is difficult to produce imagers that are free of such defects.
One way of effectively increasing the yield of imagers is to substitute a corrected signal component for that produced by any individual defective element of an imager. However, before such a correction can take place, it is necessary to detect those elements of the imager which are defective.
One proposed solution to this problem involves (1) analyzing the output signal from an imager, in the absence of an image, to detect the location of each then-existing defective element, (2) permanently storing the location of each defective element in a memory which is associated with the imager, and (3) during subsequent use of the imager, substituting a corrected signal for the signal produced by each detective element as determined by its location stored in the memory. This proposed solution may be impractical because many defects are temperature dependent. Therefore, defective elements present during subsequent use of the imager may not have been present at the time of analysis and storage of the location of defective elements in the memory. In any case, the requirement for a memory associated with the imager increases the cost of a solid-state television camera employing this proposed solution to the problem of detective imager elements.
Many systems exist which detect defective pixels in an imager array with reference to a test pattern or other fixed image. For examples, dark current defects in an imager may be detected by analyzing the image obtained when the camera's lens is capped. Similarly, defects that cause dark pixels may be detected by imaging a flat white test card. Errors detected by these methods must be stored so that they may be corrected when the camera captures active video images.
U.S. Pat. No. 4,253,120 to Peter A. Levine, entitled DEFECT DETECTION MEANS FOR CHARGE TRANSFER IMAGER, describes circuitry for determining and correcting defects in the pixels of an x-y imager using active video data. In the system described by this patent, a low-resolving power optical element is used to focus an image onto an imager that has a higher resolution than the resolving power of the optics. The difference between the resolution of the imager and the resolving power of the lens is used to distinguish errors in the imager array from transitions in the image. The referenced patent describes a charge-coupled device (CCD) imager array, which is formed on one substrate, and a correction circuit, which is formed on a separate substrate. Because the imager and the correction circuit are on separate substrates, the device is difficult to make, requiring two integrated circuit fabrications, and is relatively large in size.